This invention relates generally to testing of electronic circuitry, and more particularly concerns insulator boards to carry spring probe assemblies used in such testing.
In order to electrically test an electronic assembly, it is necessary to interconnect test points or circuits to appropriate connections on a test set. In testing printed circuit boards and other wired assemblies, this interconnection is usually accomplished through a "probing" operation. In its simplest form, probing is accomplished when a wire or probe is placed on a desired test point and a small amount of pressure is applied. Electrical contact is established and current flows through the probe to the test set and test results are observed.
In order to assure cost effective use of automated test equipment (ATE), it is often required to make hundreds or thousands of test point interconnections within a few seconds. This is accomplished through use of small device called a spring probe. As its name implies, a spring probe contains a spring and is used to probe electronic circuits. Basically, the spring mechanism allows the probe tip or plunger to be slightly displaced into the body of the probe when the test object is placed against the probe tip. Such spring action accomplishes two things:
(1) The spring tension applies the proper amount of force (2 to 4 oz. for example) to establish and maintain electrical connection with the contacted surface, and PA1 (2) The probe tip displacement (0.125 inch for example) compensates for small irregularities in the surface area contacted. PA1 (1) Automatic NC drill machines cannot be used, as long, small diameter drills tend to bend or bow especially at drill speeds of automatic drill machines. A manual drill press is used instead, and fabrication costs increase very rapidly. PA1 (2) Accuracy of hole locations using a manual drill press is very poor and frequently unacceptable. The inaccuracies of spot drilling are compounded by accumulated errors in using a travel dial across a wide (18") probe field. PA1 (3) Small diameter, long length drills tend to wander or "drift" even at extremely slow drill rates. Hole diameters, therefore, are inaccurate and cause probe misalignment. PA1 (a) upper and lower horizontal plates which are vertically spaced apart, PA1 (b) vertically extending tubular housings to receive spring probe assemblies, the housings extending through bores formed in the plates and the housings operatively connected to the plates, and PA1 (c) insulative material between the plates and bonded to intermediate portions of the housings between the plates. PA1 (a) Construction cost is drastically reduced as a result of automation in the hole drilling process and lower material cost. PA1 (b) Insulation resistance and structural strength are improved through use of the aluminum and epoxy fiberglass materials. PA1 (c) Configuration board thickness is reduced to about 1/16" for hole drilling purposes. PA1 (d) Weight is approximately 1/4 that of conventional designs. PA1 (e) Center line spacing of holes can be as low as 0.025" without affecting cost or production since holes are automatically drilled through thin PCB material.
As can be seen, any number of spring probes can be fixtured to any desired pattern and all can be engaged or disengaged through a single operation.
Spring probes are typically mounted in an insulator board in a pattern that corresponds to the geometry of the test object. Each desired test point has a probe and all probes are wired through a connector arrangement to a test set. The insulator board, spring probes and associated wiring are referred to as a "test fixture" because the pattern is fixed to the geometry of a particular test object. In general, insulator boards are removeable within a fixture so that multiple products can be tested with a common test set.
In the present art, insulator boards are constructed from heavy duty 1/2" to 1" phenolic material. Drilling small diameter holes in thick insulation material to receive spring probe assemblies creates three basic problems:
Excessive weight of such thick insulator boards is a further problem. Larger fixtures (24".times.24" probe field) pose problems in operation as previously mentioned and create safety and damage hazards as well. In test operations, a fork lift is often times required to change probe heads or insulator boards. There is need for a comparable size insulator board which will weigh only approximately 25 to 30 pounds.
A further problem concerns the use of phenolic materials which have come into disfavor in the past few years because of their tendency to absorb moisture. This decreases leakage resistance between probes and causes insulator boards to expand and/or warp.